Convert between Markdown elements

Question

What are the options to parse Markdown document and process its elements to output an another Markdown document?

Let's say it

```
# unaffected #
```

# H1 #

H1
==

## H2 ##

H2
--

### H3 ###

should be converted to

```
# unaffected #
```

## H1 ##

H1
--

### H2 ###

### H2 ###

#### H3 ####

in Node environment. Target element may vary (e.g. #### may be converted to **).

The document may contain other markup elements that should remain unaffected.

How it can be obtained? Obviously, not with regexps (using regexp instead of full-blown lexer will affect # unaffected #). I was hoped to use marked but it seems that it is capable only of HTML output, not Markdown.

Answer 1

Here is a solution with an external markdown parser, pandoc. It allows for custom filters in haskell or python to modify the input (there also is a node.js port). Here is a python filter that increases every header one level. Let's save that as header_increase.py.

from pandocfilters import toJSONFilter, Header

def header_increase(key, value, format, meta):
    if key == 'Header' and value[0] < 7:
        value[0] = value[0] + 1
        return Header(value[0], value[1], value[2])

if __name__ == "__main__":
    toJSONFilter(header_increase)

It will not affect the code block. However, it might transform setex-style headers for h1 and h2 elements (using === or ---) into atx-style headers (using #), and vice-versa.

To use the script, one could call pandoc from the command line:

pandoc input.md --filter header_increase.py -o output.md -t markdown

With node.js, you could use pdc to call pandoc.

var pdc = require('pdc');
pdc(input_md, 'markdown', 'markdown', [ '--filter', './header_increase.py' ], function(err, result) {
  if (err)
    throw err;

  console.log(result);
});